Systems and methods for seismic data acquisition employing asynchronous, decoupled data sampling and transmission

ABSTRACT

Systems and methods for asynchronously acquiring seismic data are described, one system comprising one or more seismic sources, a plurality of sensor modules each comprising a seismic sensor, an A/D converter for generating digitized seismic data, a digital signal processor (DSP), and a sensor module clock; a seismic data recording station; and a seismic data transmission sub-system comprising a high precision clock, the sub-system allowing transmission of at least some of the digitized seismic data to the recording station, wherein each sensor module is configured to periodically receive from the sub-system an amount of the drift of its clock relative to the high precision clock. This abstract is provided to comply with rules requiring an abstract to ascertain the subject matter of the disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 CFR 1.72(b).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/683,883, filed Mar. 8, 2007, which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to the field of seismic data acquisitionsystems and methods of using same. More specifically, the inventionrelates to systems and methods for seismic data acquisition in which theseismic sampling is decoupled from data transmission using asynchronousdigital signal processors for data sampling, and interpolation forsynchronizing the sampling.

2. Related Art

Land seismic acquisition aims to capture the acoustic and elastic energythat has propagated through the subsurface. This energy may be generatedby one or more surface sources such as vibratory sources (vibrators).The vibrators produce a pressure signal that propagates through theearth into the various subsurface layers. Here elastic waves are formedthrough interaction with the geologic structure in the subsurfacelayers. Elastic waves are characterized by a change in local stress inthe subsurface layers and a particle displacement, which is essentiallyin the same plane as the wavefront. Acoustic and elastic waves are alsoknown as pressure and shear waves. Acoustic and elastic waves arecollectively referred to as the seismic wavefield.

The structure in the subsurface may be characterized by physicalparameters such as density, compressibility, and porosity. A change inthe value of these parameters is referred to as an acoustic or elasticcontrast and may be indicative of a change in subsurface layers, whichmay contain hydrocarbons. When an acoustic or elastic wave encounters anacoustic or elastic contrast, some part of the waves will be reflectedback to the surface and another part of the wave will be transmittedinto deeper parts of the subsurface. The elastic waves that reach theland surface may be measured by motion sensors (measuring displacement,velocity, or acceleration, such as geophones, accelerometers, and thelike) located on the land. The measurement of elastic waves at the landsurface may be used to create a detailed image of the subsurfaceincluding a quantitative evaluation of the physical properties such asdensity, compressibility, porosity, etc. This is achieved by appropriateprocessing of the seismic data.

Seismic sensor units typically also contain the electronics needed todigitize and record the seismic data. In one known embodiment, eachsensor unit is connected to a land seismic cable, which is connected viacables to a recording instrument on a surface vehicle or other surfacefacility such as a platform. The land seismic cable provides electricpower and the means for transferring the recorded and digitized seismicsignals to the recording instrument. In other embodiments, there havebeen efforts to reduce the use of cables in performing land seismic,with movement toward wireless land seismic systems and methods.

Seismic sampling in a typical seismic sensor network (whether wired orwireless) may comprise up to tens of thousands or more seismic sensorsmeasuring the seismic vibrations for oil and gas exploration. Eachsensor with an analogue output has its output converted to a digitalsignal by an analog to digital converter (ADC) that is in turn connectedto a digital signal processing (DSP) unit. Every sampling unit has itsown clock frequency that drifts over time relative to the datatransmission line clock that may assumed to be the master clock. Thedigital data is typically transmitted to a centralized recording unit.The individual sampling ADC/DSP units are traditionallyphase-synchronized to the data transmission line clock by an electronicphase-locked loop (PLL).

While these systems and methods have enjoyed some success, there remainsroom for improvement. It is of utmost important in seismic acquisitionto phase synchronize the sampling of all the sampling units. However,presently known systems and methods are more expensive and less flexibledue to the above-mentioned individual sampling ADC/DSP units beingphase-synchronized to the data transmission line clock by an electronicphase-locked loop. There is a need in the seismic data acquisition artsfor systems and methods wherein the transmission of data is decoupledfrom sampling of the data, and that eliminate the costly and inflexibleelectronic phase locking loop, while still ensuring that the outputsampling frequency of each signal processing unit is phase synchronizedwith the data transmission line clock. The present invention is devotedto addressing one or more of these needs.

SUMMARY OF THE INVENTION

In accordance with the present invention, systems and methods forseismic data acquisition are described which reduce or overcomeshort-comings of previously known systems and methods wherein thetransmission of data is coupled to sampling of the data. Systems andmethods of seismic data acquisition in accordance with the inventioneliminate the costly and inflexible electronic phase locking loop. Inthe inventive systems and methods, the drift of each clock associatedwith a seismic sensor is periodically measured and/or calculatedrelative to the data transmission line clock (which may be the masterclock), and interpolation techniques are used to adjust for the sensorclock drift. In this way the output sampling frequency of each signalprocessing unit is phase synchronized with the data transmission lineclock without the use of an electronic phase locked loop circuit.Systems and methods of the invention allow more efficient seismic dataacquisition, for example 2-D, 3-D and 4-D land seismic data acquisition,such as during exploration for underground hydrocarbon-bearingreservoirs, or monitoring existing reservoirs. Electromagnetic signalsmay be used to transfer data to and/or from the sensor units, totransmit power, and/or to receive instructions to operate the sensorunits.

A first aspect of the invention is seismic data acquisition systemcomprising:

-   -   one or more seismic sources (which may be land sources, such as        vibrators, explosive charges, and the like, or marine sources,        such as air-guns, vibrators, and the like);    -   a sensor system (which may be suitable for land seismic or        marine seismic) for acquiring and/or monitoring analog seismic        sensor data, the sensor system comprising a plurality of sensor        modules each configured to asynchronously sample seismic data        and comprising a seismic sensor, an A/D converter (ADC) for        generating digitized seismic data, a digital signal processor        (DSP), and a sensor module clock;    -   a seismic data recording station, and    -   a seismic data transmission sub-system comprising a high        precision clock, the sub-system allowing the DSP to transmit at        least some of the digitized seismic data to the recording        station,

wherein each sensor module receives periodically from the sub-system anamount of the drift of its clock relative to the data transmission linehigh precision clock.

Alternatively, in certain system embodiments of the invention, ratherthan each sensor module periodically receiving an amount of drift fromthe data transmission sub-system, each DSP may periodically receiveinformation from outside the system (for example via GPS) to calculateits clock drift. In yet other system embodiments, both techniques may beemployed.

Systems of the invention may comprise each DSP in the sensor systemupsampling data at a particular fixed sampling rate relative to the highprecision clock. The data is upsampled using a linear or nonlinearinterpolation technique, based on the amount of drift of each sensormodule clock relative to the data transmission line high precisionclock, to increase its effective sampling rate. The data may then bedecimated (downsampled to a fixed sampling frequency) relative to thehigh precision clock. The period between intermittent adjustments of thesampling frequency to the high frequency clock may be determined basedon the nominal drift of the sensor module clocks, for example 50 partsper million (ppm), and the level of noise allowed in the system.

Optionally, the data transmission sub-system allows transmission of datato one or more base stations, which in turn transmit at least some ofthe data they receive to the recording station, which may beadvantageous in wireless systems and methods of the invention. Wirelessversions of systems of the invention may be characterized as comprisinga wireless data network, wherein the wireless data network comprises theseismic sensors transmitting at least a portion of the data to one ormore base stations via first wireless links which in turn transmit atleast some data they receive to the recording station via secondwireless links (for a completely wireless system), or through cables,wires, or optical fibers in other embodiments (partially wireless). Alsoas further explained herein, the recording station need not be on land,and need not be immobile. For example, the recording station may beselected from a stationary land vehicle, a moving land vehicle, astationary marine vessel, a moving marine vessel, and a moving airbornevessel, such as a helicopter, dirigible, or airplane.

Base stations, if used in wireless or partially wireless systems, may belocated strategically to cover predefined groups of sensor modules. Inthese embodiments, each group of sensor modules may relay datawirelessly via a mesh topology and/or in a hop to hop fashion (alsoreferred to herein as multi-hopping). Star topologies and othertopologies may also be used, but mesh topology will produce the greatestredundancy. Between each base station and the data recording station(for example recording truck), seismic data may be transferred directlyfrom base station to recording station. Sensor modules may be spacedrelatively close together in systems of the invention, for example adistance ranging from 1 meter up to about 10 meters. Because of therelatively short distance between sensor modules, multi-hopping maycircumvent the potential wireless communication (RF, microwave,infra-red) problems in uneven terrain, or terrain including man-madeobstacles. It is known that for transmitting data wirelessly betweenpoints A and B separated by a large distance, relaying between multiplespots between A and B will consume less energy compared to directwireless communication between points A and B.

Systems within the invention include those comprising a first wirelesslink that wirelessly transmits seismic data sampled from a seismicsensor to a base station (which may be a mobile or non-mobilecommunication device), the base station having a second wireless linkthat receives the seismic data from the sensor modules and wirelesslytransmits the seismic data to the land seismic data recording station,the one or more vibrators having a third wireless link that receivescommands from the land seismic data recording station and wirelesslytransmits vibrator data (such as status information) to the land seismicdata recording station. As used herein the term “mobile”, when used todescribe a device, includes hand-held devices and devices that may beworn on the body of a person, for example on a belt, in a pocket, in apurse, and the like. It is not meant to include objects that may in factbe moved, but only with great effort, such as a building or shed, orwith less effort a desk top computer.

In certain system embodiments the first wireless link may be selectedfrom any wireless personal area network (WPAN) communication protocol.The second and third wireless links may be individually selected fromany wireless communication protocol that supports point to multi-point(PMP) broadband wireless access. These protocols may include, but arenot limited to IEEE standard 802.16 (sometimes referred to as the WiMax(Worldwide Interoperability for Microwave Access) standard), IEEEstandard 802.20, and the like. The second and third wireless links mayuse the same or different protocols.

Certain land seismic data acquisition systems of the invention mayutilize wireless links and equipment allowing broadcasting of messages(audio, video, alphanumeric, digital, analog, and combinations thereof)between sensor modules, vibrators, base stations, and the recordingstation, or simply between the sensor modules. The messages may be timetagged and used for distance measure and clock calibration. Thecommunication network may also be used for transmission of statusinformation and/or quality control (QC).

A second aspect of the invention comprises methods of acquiring seismicdata during a seismic survey, including time-lapse (4-D) seismic dataacquisition, one method comprising:

-   -   a) initiating one or more seismic sources;    -   b) asynchronously acquiring reflected analog seismic data using        a sensor system, the sensor system comprising a plurality of        sensor modules each comprising a seismic sensor, an A/D        converter (ADC) for generating digitized seismic data, a digital        signal processor (DSP), and a sensor module clock;    -   c) transmitting at least some of the digitized seismic data to a        data recording station via a data transmission sub-system        comprising a high precision clock having a high precision clock        frequency; and    -   d) correcting clock frequency drift of each sensor module clock        relative to the data transmission line high precision clock        frequency, and each sensor module receiving periodically from        the sub-system an amount of the drift of its clock relative to        the data transmission line high precision clock.

Other methods of the invention include passive listening surveys (whereno vibratory source is used) and electromagnetic (EM) surveys, where oneor more of the sensor units comprises one or more EM sensors.

As used herein, “survey” refers to a single continuous period of seismicdata acquisition (which may occur simultaneously, sequentially, or withsome degree of time overlap), over a defined survey area; multiplesurveys means a survey repeated over the same or a same portion of asurvey area but separated in time (time-lapse, sometimes referred toherein as 4-D seismic). In the context of the present invention a singleseismic survey may also refer to a defined period of seismic dataacquisition in which no controlled seismic sources are active (whichalso may be referred to alternatively as passive seismic listening ormicro seismic measurements).

Systems and methods of using systems of the invention allow moreefficient data acquisition (including time-lapse) than previously knownsystems and methods. These and other features will become more apparentupon review of the brief description of the drawings, the detaileddescription of the invention, and the claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

The manner in which the objectives of the invention and other desirablecharacteristics may be obtained is explained in the followingdescription and attached drawings in which:

FIG. 1 illustrates a simplified plan view of a system of the invention;

FIG. 2 illustrates schematically wireless communication betweencomponents of systems of the invention;

FIGS. 3-4 illustrate schematically prior art communication topologiesuseful in practicing systems and methods of the invention; and

FIG. 5 illustrates the protocol structure of the IEEE 802.16 Broadbandwireless MAN standard.

It is to be noted, however, that the appended drawings are not to scaleand illustrate only typical embodiments of this invention, and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

DETAILED DESCRIPTION

In the following description, numerous details are set forth to providean understanding of the present invention. However, it will beunderstood by those skilled in the art that the present invention may bepracticed without these details and that numerous variations ormodifications from the described embodiments may be possible.

As noted in the literature and on the Internet (see for example, thewebsite of Prof. Bengt Oelmann, Mid-Sweden University, Sundsvall,Sweden, accessed Dec. 10, 2006) digital designs can be divided intosynchronous and asynchronous circuits. The common timing referencecalled clock signal defines the synchronous designs. Consequently,asynchronous designs are those without a common timing reference. In theearly days of digital design, design methodologies were not establishedand combinations of synchronous and asynchronous techniques were used.From the 1960s to the present, the usage and the development ofsynchronous circuits and methods achieved almost total dominance. Whencomputers were first constructed, a few of them were fully asynchronous.Two examples are ORDVAC from the University of Illinois (1951-52) andlater, MU5 from the University of Manchester (1969-74). But asynchronoustechniques have later found their applications in places wheresynchronous techniques are not feasible. A typical example is high-speedcommunication over long distances, such in computer bus systems. UNIBUSin PDP-11 (1969) and VMEBUS (1980) are examples of such asynchronousbuses.

The growing complexity of ICs makes clock distribution in synchronousdesigns more costly to design in terms of power consumption, area, anddesign effort. The clock distribution problem has made asynchronousdesign techniques a viable alternative. Some of the research carried outin the field of asynchronous design searches for ways to utilize theseadvantages for solving real-world problems. Most of the work in thisarea is carried out at universities, but there is also some research inindustry. For example, Sun MicroSystems Labs proposed a new processorarchitecture known under the trade designation “Counterflow PipelineProcessor”, and Philips Research Labs has focused on designing low-powerICs using automatic synthesis of asynchronous circuits. In recent yearsa mixed synchronous/asynchronous approach, commonly referred to asGlobally Asynchronous—Locally Synchronous (GALS), has been advocated(according to Oelmann). The basic idea is to have a local clock for eachmodule on the chip and to have asynchronous communication between thesynchronous modules. When considering other complex digital systemsbased on multiple ICs and PCBs, this seems to be a natural developmentfor very complex ICs.

The systems and methods of the present invention offer one or more ofthe possible advantages discussed by Oelmann, and, as noted by at leastthis reference, asynchronous design methods differ significantly fromthe methods that are currently used. Some of the possible advantageousof asynchronous systems are the following:

Average case performance: In synchronous systems, the slowestcombinational path defines the maximum clock frequency. This leads toworst-case performance for all operations independently on the data.Asynchronous data-paths are designed to indicate when computation iscompleted. The computation time for many operations is very datadependent and this property can be exploited in cases where theworst-case delay is much larger than the average delay.

No or reduced clock skew problems: In a synchronous system, thedifferences in arrival time of the clock signal to different parts ofthe system must be controlled. Clock skew affects speed performance andmay also cause malfunctioning due to race conditions. The cost ofmaintaining low clock skew becomes higher when the complexity of the ICincreases. Asynchronous circuits do not have a global clock signal andclock skew is therefore not a problem.

Low power consumption: Only active parts of a CMOS design dissipatespower in CMOS. In a synchronous system, the clock signal is still activein the idle parts. The event-driven nature of asynchronous designs leadsto the fact that only the parts of the design that actually take part inthe computation are dissipating power.

Low noise: Simultaneous switching in CMOS leads to high currenttransitions in the power lines. In a synchronous system, the charge anddischarge of the clock net is a large contributor to the currenttransitions. Most of the switching in the gates occurs shortly after theactive clock edge. This makes the total current concentrated to the timeof the active clock edge. The fast current transitions causefluctuations on the power supply lines that may cause lowered speedperformance or malfunctioning of the digital logic. In a mixedanalog/digital system, the digital noise may affect sensitive analogcircuits. Asynchronous circuits are not synchronized and the current ismore uniformly distributed in time.

Modularity: In asynchronous modules, both timing and functionality maybe located inside the module. From the user's point of view, only thesequence of operations is important when using the module. Incrementalupgrading of the performance of the asynchronous system only requiresthe replacement of the module that is limiting the performance, withouthaving to change or retime the system in any other way.

Scalability: In general, a digital system consists of different partsimplemented in different technologies and these may be communicatingover different types of media. Different types of design techniques arethen used for different types of implementation technologies. A typicalscenario is as follows: Inside the IC, a high-speed global clock signalis used that is generated from a phase-locked loop (PLL), which issynchronized to a slower external clock signal. Communication betweenthe ICs on the same PCB is synchronized to the slower clock.Board-to-board communication is handled by an asynchronous standard bussystem (such as VMEBUS). Crossing the boarders of implementationtechnologies makes it necessary to introduce new design techniques. Byusing asynchronous circuits from the beginning, it is possible to keepthe same design technique throughout the system design.

As discussed in U.S. Pat. No. 6,049,882, “synchronous” systems apply afixed time step signal (i.e., a clock signal) to the functional units toensure synchronized execution. Thus, in synchronous systems, all thefunctional units require a clock signal. However, not all functionalunits need be in operation for a given instruction type. Since thefunctional units can be activated even when unnecessary for a giveninstruction execution, synchronous systems can be inefficient.

The use of a fixed time clock signal (i.e., a clock cycle) insynchronous systems also restricts the design of the functional units.Each functional unit must be designed to perform its worst caseoperation within the clock cycle even though the worst case operationmay be rare. Worst case operational design reduces performance ofsynchronous systems, especially where the typical case operationexecutes much faster than that of the worst case criteria. Accordingly,synchronous systems attempt to reduce the clock cycle to minimize theperformance penalties caused by worst case operation criteria. Reducingthe clock cycle below worst case criteria requires increasingly complexcontrol systems or increasingly complex functional units. These morecomplex synchronous systems reduce efficiency in terms of area and powerconsumption to meet a given performance criteria such as reduced clockcycles.

In asynchronous seismic data acquisition systems and methods of theinvention, performance penalties only occur in an actual (rare) worstcase operation, and the inventive systems and methods may be tailoredfor typical case performance, which can result in decreased complexityfor processor implementations that achieve the performance requirements.Further, because asynchronous systems only activate functional unitswhen required for the given instruction type, efficiency is increased.Thus, the asynchronous seismic data acquisition systems and methods ofthe invention may provide increased efficiency in terms of integrationand power consumption.

A first aspect of the invention is seismic data acquisition systemcomprising:

-   -   one or more seismic sources (which may be land sources, such as        vibrators, explosive charges, and the like, or marine sources,        such as air-guns, vibrators, and the like);    -   a sensor system (which may be suitable for land seismic or        marine seismic) for acquiring and/or monitoring analog seismic        sensor data, the sensor system comprising a plurality of sensor        modules each configured to asynchronously sample seismic data        and comprising a seismic sensor, an A/D converter (ADC) for        generating digitized seismic data, a digital signal processor        (DSP), and a sensor module clock;    -   a seismic data recording station, and    -   a seismic data transmission sub-system comprising a high        precision clock, the sub-system allowing the digital signal        processor to transmit at least some of the digitized seismic        data to the recording station,

wherein each sensor module clock has a clock frequency that drifts overtime relative to a data transmission line high precision clockfrequency, and each sensor module receives periodically from thesub-system an amount of the drift of its clock relative to the datatransmission line high precision clock.

Alternatively, in certain system embodiments of the invention, ratherthan each sensor module periodically receiving an amount of drift fromthe data transmission sub-system, each DSP may periodically receiveinformation from outside the system (for example via GPS) to calculateits clock drift. In yet other system embodiments, both techniques may beemployed.

Systems of the invention may comprise each DSP in the sensor systemupsampling data to a particular fixed sampling rate relative to the highprecision clock, and based on this drift using linear or nonlinearinterpolation to increase its effective sampling rate. The data may thenbe downsampled to a fixed sampling frequency relative to the highprecision clock. The period between intermittent adjustments of thesampling frequency to the high frequency clock may be determined basedon the nominal drift of the sensor module clocks, for example 50 partsper million (ppm), and the level of noise allowed in the system.

Decimation is the process of filtering and downsampling a signal todecrease its effective sampling rate. The filtering is employed toprevent aliasing that might otherwise result from downsampling. Theoperation of downsampling by factor Mε N describes the process ofkeeping every Mth sample and discarding the rest. This is denoted by“↑M” in block diagrams. Interpolation is the process of upsampling andfiltering a signal to increase its effective sampling rate. Theoperation of upsampling by factor LεN describes the insertion of L−1zeros between every sample of the input signal. This is denoted by “↑L”in block diagrams.

Systems and methods of the invention may operate as follows: we canassume that in the DSP at one stage we have a sampling frequency of X+δHz, where “X” is the nominal sampling frequency and “δ” is the DSPclock's deviation from the data transmission line clock or the masterclock. In certain embodiments of the invention 6 is calculated andprovided periodically to the DSP, for example by the recording station.The sampling frequency, X, is then interpolated to MX, where “M” is aninteger. An example may clarify this procedure. In one example at oneparticular decimation stage in a DSP we assume a sampling frequency of20 kHz+δ relative to the master clock's sampling frequency. δ is smallhere. In this particular example we apply a linear interpolation of“approximately” 16 times (i.e., M=16) to 320 kHz. The signal is thendownsampled, this time “exactly” 16 times, back to 20 kHz by adownsampling filter and prior to further filtering and downsampling atlater decimation stages. The upsampling factor used in interpolation,linear or nonlinear, is largely determined by the level of noise allowedat the output.

Suitable interpolation techniques include any method of constructing newdata points from a discrete set of known data points. There are manydifferent interpolation methods, such as linear, polynomial, spline, andthe like. Some of the concerns to take into account when choosing anappropriate interpolation algorithm are how accurate is the method, howexpensive is it, how smooth is the interpolant, and how many data pointsare needed. Linear interpolation is generally easier to implement thanother interpolation methods, but may not have the desired accuracy.Error is proportional to the square of the distance between the datapoints. Another disadvantage is that the interpolant is notdifferentiable at the point of interest. Polynomial interpolation is ageneralization of linear interpolation. In linear interpolation, theinterpolant is a linear function. In polynomial interpolation the linearinterpolant is replaced by a polynomial of higher degree. Theinterpolation error is proportional to the distance between the datapoints to the power n, where n is the number of known data points.Furthermore, the interpolant is a polynomial and thus infinitelydifferentiable. However, polynomial interpolation also has somedisadvantages. Calculating the interpolating polynomial may berelatively computationally expensive. Furthermore, polynomialinterpolation may not be exact at the end points. These disadvantagescan be avoided by using spline interpolation. Spline interpolation useslow-degree polynomials in each of the intervals, and chooses thepolynomial pieces such that they fit smoothly together. The resultingfunction is called a spline. Other forms of interpolation may be used inthe systems and methods of the invention by picking a different class ofinterpolants. Some examples include rational interpolation, which isinterpolation by rational functions, and trigonometric interpolation,which is interpolation by trigonometric polynomials. The discreteFourier transform is a special case of trigonometric interpolation.Another possibility is to use wavelets. Multivariate interpolation isthe interpolation of functions of more than one variable, and suchmethods include bilateral interpolation and bicubic interpolation in twodimensions, and trilateral interpolation in three dimensions.

Systems and methods of the invention may be used in land- andmarine-seismic surveying, and may employ wired (copper wires or opticalfiber connections) or wireless transmission of data and commands. In anycase, the sampling is decoupled from the data transmission. Each DSP/ADCunit has its own clock that is not phase-locked by an electronic device(electronic PLL) to the data transmission line. Stated differently, thesystems and methods of the invention, by decoupling the transmission ofdata from sampling, and avoidance of the need for an electronic PLL,allows the systems and methods of the invention to be less expensive,more robust and flexible for seismic data acquisition systems.

Digital signal processors useful in the invention may be either fixed orfloating-point DSPs, and are available from a number of suppliers,including Texas Instruments, Analog Devices, Lucent Technologies,Infineon, and Philips. Programming of fixed or floating-point DSPs maybe accomplished using a number of techniques, ranging from programmingdirectly using assembly language, which may be difficult, to programminga higher order code, such as C language, or an object-oriented languagesuch as C++ language, and then using a suitable compiler.

Systems and methods of the invention may be “completely wireless”,wherein all wires, cables, and fibers for communication betweenvibrators, seismic sensors, base stations, and the recording station aresubstantially eliminated. This does not rule out the use of wires,cables, or fibers (such as optical fibers) for example in the recordingstation equipment and vibrators, for example for power, and the use oftie-down cables if necessary in windy conditions. In marine systems,this does not rule out towing cables, distance cables, and the like,required to deploy the seismic sources and sensors, deflectors, and thelike.

Wireless systems and methods may offer improvements over systems andmethods that use wire or optical fiber for communications in terms ofone or more of robustness, scalability, cost, and power-efficiency.Systems and methods of the invention allow more efficient seismic dataacquisition, for example 3-D and 4-D land seismic data acquisition, suchas during exploration for underground hydrocarbon-bearing reservoirs, ormonitoring existing reservoirs. Electromagnetic signals may be used totransfer data to and/or from the sensor units, to transmit power, and/orto receive instructions to operate the sensor units.

A simplified schematic view of a land seismic data acquisition system ofthe invention is illustrated in FIG. 1. An area 2 to be surveyed, mayhave physical impediments to direct wireless communication between, forexample, a recording station 14 (which may be a recording truck) and avibrator 4 a. A plurality of vibrators 4 a, 4 b, 4 c, 4 d may beemployed, as well as a plurality of sensor unit grids 6 a, 6 b, 6 c, 6d, 6 e, and 6 f, each of which may have a plurality of sensor units 8.As illustrated in FIG. 1, for example approximately 24-28 sensor units 8may be placed in the general vicinity around a base station 10. Thenumber of sensor units 8 associated with each base station 10 may varywidely according to the goals of the survey number, however, due to thearchitecture of the communications between the various components(discussed herein, particularly with reference to FIGS. 3 and 4), thenumber should be less than required in previously known systems. Circles12 indicate the approximate range of reception for each base station 10.This range may be the same or different for each base station.

The system illustrated in FIG. 1, using the plurality of sensor units 8,may be employed in acquiring and/or monitoring land-seismic sensor datafor area 2, and transmitting the data to the one or more base stations10. All communications between vibrators 4, base stations 10, recordingstation 14, and seismic sensors 8 are completely wireless, as that termis defined herein. Alternatively, systems of the invention may bedescribed as comprising a wireless data network, for example asillustrated schematically in FIG. 2, wherein the wireless data networkcomprises multiple seismic sensors 8 transmitting at least a portion ofthe seismic data they sense to the one or more base stations 10 viafirst wireless links 9, which in turn transmit at least some data theyreceive to the recording station 14 via second wireless links 16.Commands may be sent from recording station 14 to vibrators 4 viawireless links 18, and, to the extent data is exchanged betweenvibrators 4 and recording station 14, wireless links 18 may also beconsidered part of the wireless data network.

First wireless links 9 may be characterized as Wireless Personal-AreaNetworks (WPAN). A “WPAN” is a personal area network (PAN) usingwireless connections. WPAN is currently used for communication amongdevices such as telephones, computers and their accessories, as well aspersonal digital assistants, within a short range. The reach of a PAN istypically within about 10 meters. These protocols may include, but arenot limited to Bluetooth (registered certification mark of BluetoothSIG, Inc., Bellevue Wash.), ZigBee (registered certification mark ofZigBee Alliance Corporation, San Ramon, Calif.), Ultra-wideband (UWB),IrDA (a service mark of Infrared Data Association Corporation, WalnutCreek, Calif., HomeRF (registered trademark of HomeRF Working GroupUnincorporated Association Calif., San Francisco, Calif.), and the like.Bluetooth is the most widely used technology for the WPAN communication.Each technology is optimized for specific usage, applications, ordomains. Although in some respects, certain technologies might be viewedas competing in the WPAN space, they are often complementary to eachother.

The IEEE 802.15 Working Groups is the organization to define the WPANtechnologies. In addition to the 802.15.1 based on the Bluetoothtechnology, IEEE proposed two additional categories of WPAN in 802.15:the low rate 802.15.4 (TG4, also known as ZigBee) and the high rate802.15.3 (TG3, also known as Ultra-wideband or UWB). The TG4 ZigBeeprovides data speeds of 20 Kbps or 250 Kbps, for home control type oflow power and low cost solutions. The TG3 UWB supports data speedsranging from 20 Mbps to 1 Gbps, for multi-media applications. In theTable 1, the main characters of the WPAN technologies as specified inthe IEEE 802.15 are delineated.

TABLE 1 Wireless Personal Area Network Characteristics Bluetooth UWBZigBee Parameters (IEEE 802.15.1) (IEEE 802.15.3) (IEEE 802.15.4)Applications Computer and accessory Multimedia content transfer, Homecontrol devices High-resolution radar, Building automation Computer tocompute Ground-penetrating radar, Industrial automation Computer withother Wireless sensor network, Home security digital devices Radiolocations systems Medical monitoring Frequency Band: 2.4-2.48 GHz3.1-10.6 GHz 868 MHz 902-928 MHz 2.4-2.48 GHZ Range ~10 meters ~10meters ~100 meters Maximum Data 3 Mbps 1 Gbps 20 Kbps transfer rate: 40Kbps 250 Kbps Modulation GFSK, 2PSK, DQSP, 8PSK OPSK, BPSK BPSK (868/928MHz) OPSK (2.4 GHz)

In wired communication systems, mesh network topology is one of the keynetwork architectures in which devices are connected with many redundantinterconnections between network nodes such as routers and switches.(See definition of mesh topology in the networkdictionary.com) In awired communication system using mesh topology, if any cable or nodefails, there are many other ways for two nodes to communicate. Whileease of troubleshooting and increased reliability are definite pluses,wired mesh networks are expensive to install because they use a lot ofcabling. Often, a mesh topology will be used in a wired communicationsystem in conjunction with other topologies (such as Star, Ring and Bus)to form a hybrid topology. Some WAN architecture, such as the Internet,employ mesh routing. Therefore the Internet allows sites to communicateeven during a war.

There are two types of mesh topologies: full mesh (as depicted in FIG.3) and partial mesh (as depicted in FIG. 4). Full mesh topology occurswhen every node has a circuit connecting it to every other node in anetwork. In wired networks, full mesh is very expensive to implement butyields the greatest amount of redundancy, so in the event that one ofthose nodes fails, network traffic can be directed to any of the othernodes. Full mesh is usually reserved for backbone networks. With partialmesh, some nodes are organized in a full mesh scheme but others are onlyconnected to one or two in the network. Partial mesh topology iscommonly found in peripheral networks connected to a full meshedbackbone. It is typically less expensive to implement and yields lessredundancy than full mesh topology.

In wireless systems and methods of the invention, due to the wirelessnature of the communications using a wireless data network architecture,redundancy, robustness, and flexibility, are all increased, while costas reduced. As illustrated in the full mesh topology of FIG. 3, sensors8 a-8 h may communicate wirelessly directly with each other sensorthrough multiple direct wireless links 20. In other embodiments, forexample as illustrated in the partial mesh topology of FIG. 4, sensor 8a may communicate wirelessly directly with only sensors 8 b, 8 c and 8 gvia wireless communications 20, and indirectly with sensors 8 d, 8 e, 8f, and 8 h though wireless communication links 22.

The second and third wireless links (i.e., links 16 and 18,respectively, as illustrated in FIG. 2) may be individually selectedfrom any wireless communication protocol that supports point tomulti-point (PMP) broadband wireless access. These protocols mayinclude, but are not limited to IEEE standard 802.16 (sometimes referredto as the WiMax (Worldwide Interoperability for Microwave Access)standard), IEEE standard 802.20, and the like. The IEEE wirelessstandard presently has a range of up to 30 miles (48 km), and presentlycan deliver broadband at around 75 megabits per second, although theinvention is not so limited. This is theoretically, 20 times faster thana commercially available wireless broadband.

The IEEE 802.16 WiMax standard was published in March 2002 and providedupdated information on the Metropolitan Area Network (MAN) technology.The extension given in the March 2002 publication extended theline-of-sight fixed wireless MAN standard, focused solely on a spectrumfrom 10 GHz to 60+ GHz. This extension provides for non-line of sightaccess in low frequency bands like 2-11 GHz. These bands are sometimesunlicensed. This also boosts the maximum distance from 31 to 50 miles(50 to 80 km) and supports PMP (point to multipoint) and meshtechnologies. The IEEE approved the 802.16 standards in June 2004. WiMaxmay be used for wireless networking like the popular WiFi. WiMax, asecond-generation protocol, allows higher data rates over longerdistances, efficient use of bandwidth, and avoids interference almost toa minimum. WiMax can be termed partially a successor to the Wi-Fiprotocol, which is measured in feet, and works over shorter distances.

As used in the context of seismic data acquisition in systems of theinvention, the seismic sensors and base stations may be compared to ametropolitan area networking (MAN), as given in the 802.16 standard,sometimes referred to as fixed wireless. In fixed wireless, a backboneof base stations is connected to a public network. As with a MAN, eachof base station 10 supports many “fixed subscriber stations” (sensorunits 8), which are akin to either public WiFi hot spots or fire walledenterprise networks. Base stations 10 use a media access control (MAC)layer, and allocate uplink and downlink bandwidth to “subscribers”(sensor units 8) as per their individual needs. This is basically on areal-time need basis. The MAC layer is a common interface that makesnetworks interoperable. In the future, one can look forward to 802.11hotspots, hosted by 802.16 MANs. These would serve as wireless localarea networks (LANs) and would serve the end users directly too.

WiMax has two main topologies, either of which may be used in systemsand methods of the present invention, namely Point to Point for backhauland Point to Multi Point Base station for Subscriber station. In each ofthese situations, multiple input multiple output antennas may be used.The protocol structure of IEEE 802.16 Broadband wireless MAN standard isillustrated in FIG. 5. FIG. 5 shows four layers: convergence, MAC,transmission and physical. These layers map to two of the lowest layers,physical and data link layers of the OSI model.

Use of WiMax protocol provides systems and methods of the invention andtheir end users many user applications and interfaces, for exampleEthernet, TDM, ATM, IP, and VLAN. The IEEE 802.16 standard is versatileenough to accommodate time division multiplexing (TDM) or frequencydivision duplexing (FDD) deployments and also allows for both full andhalf-duplex terminals.

IEEE 802.16 supports three physical layers. The mandatory physical modeis 256-point FFT OFDM (Orthogonal Frequency Division Multiplexing). Theother modes are Single carrier (SC) and 2048 OFDMA (Orthogonal FrequencyDivision Multiplexing Access) modes. The corresponding Europeanstandard—the ETSI Hiperman standard defines a single PHY mode identicalto the 256 OFDM modes in the 802.16d standard.

The MAC was developed for a point-to-multipoint wireless accessenvironment and can accommodate protocols like ATM, Ethernet and IP(Internet Protocol). The MAC frame structure dynamic uplink and downlinkprofiles of terminals as per the link conditions. This entails atrade-off between capacity and real-time robustness. The MAC uses aprotocol data unit of variable length, which increases the standardsefficiency. Multiple MAC protocol data unit may be sent as a singlephysical stream to save overload. Also, multiple Service data units(SDU) may be sent together to save on MAC header overhead. Byfragmenting, large volumes of data (SDUs) may be transmitted acrossframe boundaries and may guarantee a QoS (Quality of Service) ofcompeting services. The MAC uses a self-correcting bandwidth requestscheme to avoid overhead and acknowledgement delays. In systems andmethods of the invention, this feature may also allows better QoShandling than previously known systems and methods. The terminals have avariety of options to request for bandwidth depending on the QoS andother parameters. The signal requirement can be polled or a request canbe piggybacked.

In systems and methods of the invention, the 802.16 MAC protocol mayperform Periodic and Aperiodic activities. Fast activities (periodic)like scheduling, packing, fragmentation and ARQ may be hard-pressed fortime and may have hard tight deadlines. They must be performed within asingle frame. The slow activities, on the other hand, may execute as perpre-fixed timers, but are not associated with any timers. They also donot have specific time frame or deadline.

Table 2 compares similarities and differences in the first wireless andsecond and third wireless links useable in systems and methods of theinvention (borrowed from Javvin.com).

TABLE 2 Comparable Properties of First and Second Wireless LinksParameters WiMax WLAN Bluetooth Frequency 2-11 GHz 2.4 GHz Varies Range~31 miles (50 km) ~100 metres ~10 metres Data Transfer 70 mbps 11-55mbps 20-55 mbps Rate Users 1000 s >10 >10

The 802.11 is based on a distributed architecture, whereas, WiMax isbased on a centrally controlled architecture. In this the schedulerresiding in the Base station (BS) has control of the wireless mediaaccess. WiMax can support multiple connections conforming to a set ofQoS parameters and provides the packet classifier ability to map theconnections to many user applications and interfaces.

Certain embodiments of systems and methods of the invention may use awireless data network based on a newer protocol, IEEE 802.20. Thisstandard, like the 802.16 standard, is aimed at wireless high-speedconnectivity to mobile consumer devices like cellular phones, PDAs andlaptop computers. The IEEE 802.20 Mobile Broadband Wireless AccessWorking Group is developing an air-interface standard for mobile BWAsystems that operate in licensed bands below 3.5 GHz. It is targetingpeak data rates of over 1 Mb/s per user at vehicular speeds to 250km/hour. This maybe useful for systems of the invention using, forexample, a moving data recoding stations, for example a moving truck, anairplane or helicopter, rather than a stationary recording station.Systems and methods using this standard will operate in the 500 MHz-3.5GHz range. Currently, this protocol is offered by QUALCOMM FlarionTechnologies, Bedminster, N.J., and ArrayComm, San Jose, Calif.

Systems and methods of the invention may include provision ofmulti-antenna signal processing (MAS) software architectures forimplementation of the second and/or third wireless links employingWiMAX. The WiMAX profiles support both adaptive antenna system (AAS) andmultiple-input/multiple-output (MIMO) architectures in baseline form.MAS implementation, such as though use of the product known under thetrade designation “A-MAS” from ArrayComm, may enhance baseline MIMOthrough the addition of essential interference mitigation. Generic MIMOsystems provide link robustness and enhance point-to-point data rates bytransmitting signals multiple times and/or transmitting multiplesignals. Without active interference mitigation, these additionaltransmissions incur the cost of decreased signal-to-interference ratiosfor co-channel users in other cells. In the single-cell environmentstypified by wireless LANs, where MIMO techniques have seen their firstcommercial success, this increased interference has no adverse effects.In a networked system such as WiMAX where multiple cells share the samespectral resources, the increased interference degrades network capacityand overall service quality, even though it may improve links for someusers. It also prevents the use of MIMO techniques for enhancing datarates outside the cell center. By combining AAS techniques with MIMO inour A-MAS solution, MIMO's benefits can be realized throughout the cellsin the network, simplifying network planning and providing performanceimprovements operators can rely on. A-MAS software may run as asynthesizable core or as an embedded DSP code within common ASICarchitectures, integrating into client device physical layers throughmodular interfaces. The approach taken by software products such a thatknown as A-MAS takes precise control of the space dimension and putsradio energy (or receive sensitivity) only where its really required.The software drives an array of two or more antennas on either theclient device, the base station, or both, leveraging the principle ofcoherent combinations of radio waves to create a focus of transmitenergy (or receive sensitivity) on the intended recipient (sender) andthe absence of energy (sensitivity) on sources of co-channelinterference. As applied in the context of inventive methods andsystems, A-MAS-enabled base stations and sensor units may take advantageof all the possible gains from using multiple antennas: link budgetimprovements from diversity and combining gains, along with client datarate and overall network capacity benefits from active interferencemitigation and spatial mutliplexing.

Systems and methods of the present invention solve or reduce problemsassociated with cable-based land seismic systems, or previously knownsensor unit-based systems for acquisition of time-lapse land seismicdata, namely cost, power and data transfer.

Land sensor units useable in the invention may include, in addition tomeasurement sensors, a high-precision clock, low-power electronics,long-term battery and memory components, and an autonomous powergenerating unit which provides power to charge the batteries in thesensor units without being reliant on power charge from external means.

The sensor units may remain on the land between seismic surveys or beremoved therefrom. During idle periods, an autonomous power generationcomponent, if present, will generate enough power to recharge theautonomous power source, which may be one or more rechargeablebatteries, one or more capacitors, and the like. Batteries andcapacitors may be based on any chemistry as long as they areself-sufficient for the duration intended, which may be months to years.Batteries or battery cells such as those known under the tradedesignation “Li-ion VL45E”, available from SAFT, Bagnolet, France, maybe used. Another alternative is to use capacitors as storage devices forpower. Capacitors are smaller and have higher storage capacity, such asdiscussed in the publication “Researchers fired up over new battery”,MIT News Office, Feb. 8, 2006, incorporated herein by reference.Furthermore, sensor units of the invention may be placed in “sleep” modefor energy conservation during periods of no operation.

“Autonomous power generation” components are to be distinguished from“autonomous power sources.” As used herein, the phrase “autonomous powergeneration” is an optional, but highly desirable feature for sensorunits of the invention, and refer to one or more components allowing theautonomous power source or sources to be regenerated, recharged, orreplenished, either fully or partially, in order that the seismic sensorunit may remain on the land between seismic surveys. While in theorythis may be possible through power brought to the seismic sensor unit bymeans of a vehicle, this is a slow and cumbersome process. Instead, thesensor units of the present invention may include a means of extractingpower from their local environment, sometimes referred to as energyharvesting. Examples of suitable autonomous power generation componentsinclude those which may use wind energy, solar energy, and the like,which may be transformed into electrical energy by known means of energyconversion. The autonomous power sources (batteries, for example) may berecharged during periods between seismic surveys which could be anywherebetween a few months and one to two years.

Sensors useable in the invention for land-based seismic may beindividual sensors or a package of two or more sensors. One suitablesensor package is that known under the trade designation “4C Sensor”available from WesternGeco LLC, comprised of three geophones oraccelerometers.

Sensor units useable in the invention may also comprise an electronicsmodule having ultra-low power requirements, and may include ahigh-precision clock, an analog-to-digital converter, power managementsoftware and hardware, and a control module for data input/output.

The total power consumption of the digitizing electronics within asensor unit may be expected to not exceed 50 m Watt. In addition, byusing low-power memory (for example flash EPROM), the total powerconsumption of the complete inventive sensor units is not expected toexceed 150 mW at any time. This is at least a factor of 10 less thanwith current technology used in land sensor units. The battery capacitythat is needed to provide power to an inventive sensor unit for atypical seismic survey period of six weeks is only 150 Wh. RechargeableLi-Ion batteries may provide approximately 350 Wh/1 and 150 Wh/kg, hencethe total battery volume and weight is expected approximately 0.41 literand 0.6 kg.

Data that is recorded by the land sensor units may be transferred to thebase stations, and in turn to the recording station. In otherembodiments it may be desirable to remove and transport one or morememory modules from a particular sensor unit. For example, one mightequip a sensor unit with N memory modules for N surveys. In theseembodiments, for example, for each survey one memory module is takenout. Both methods of data transfer may be used. In certain embodimentsdata transfer may be achieved through multiple channels and/or bymultiple methods in order to increase the speed and/or amount of thedata transmission.

Methods of using systems of the invention may include measurement,calculation and other sub-systems useful in implementing methods of theinvention. Calculation units may include software and hardware allowingthe implementation of one or more equations, algorithms and operationsas required, as well as access databases, data warehouses and the like,via wire or wireless transmission.

The initial position to within few meters of accuracy of one or moresensor units of the invention may be determined for instance by usingGPS.

The performance of a marine seismic acquisition survey typicallyinvolves one or more vessels towing at least one seismic streamerthrough a body of water believed or known to overlie one or morehydrocarbon-bearing formations. WesternGeco L.L.C., Houston, Tex.,currently conducts high-resolution Q-Marine™ surveys, in some instancescovering many square kilometers. A survey vessel known as aO-Technology™ vessel may conduct seismic surveys towing multiple,1000-12000-meter cables with a separation of 25-200 meters, using theWesternGeco proprietary calibrated Q-Marine™ source. “Q” is theWesternGeco proprietary suite of advanced seismic technologies forenhanced reservoir location, description, and management. For additionalinformation on Q-Mariner“ ”, a fully calibrated, point-receiver marineseismic acquisition and processing system, as well as Q-Land™ andQ-Seabed™. The seismic vessel and streamers progress forward at about 5knots and the system is able to cover large areas of open oceanrelatively efficiently. Thus, the traditional towed streamer seismicacquisition system is well-suited to explore the geological structuresof previously unexplored or unexploited areas.

It is within the invention to interface systems of the invention withother data acquisition systems and methods, for example other landseismic data acquisition systems, such as cable-based systems, andsystems using previously known land seismic systems. As one non-limitingexample, where a reliable land cable has been operating successfully,one might use that land cable and its sensors, and position sensor unitsin a grid on one or both sides of the cable.

In certain embodiments, regardless of the environment or survey area, ahigher density of land sensor units throughout the spread may improveoverall operational efficiency by decreasing the distances between thesensor units and the associated degradation of wireless signals. Theshape of the sensor units or grids of sensor units is not in itselfrelevant.

Although only a few exemplary embodiments of this invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention as defined inthe following claims. In the claims, no clauses are intended to be inthe means-plus-function format allowed by 35 U.S.C. §112, paragraph 6unless “means for” is explicitly recited together with an associatedfunction. “Means for” clauses are intended to cover the structuresdescribed herein as performing the recited function and not onlystructural equivalents, but also equivalent structures.

What is claimed is:
 1. A seismic data acquisition system comprising: oneor more seismic sources; a plurality of sensor modules configured toasynchronously sample seismic data, wherein respective sensor modulescomprise an analog seismic sensor, an A/D converter (ADC) for generatingdigitized seismic data, a digital signal processor (DSP), and a sensormodule clock; a seismic data recording station; and a seismic datatransmission sub-system comprising a high precision clock, thesub-system allowing transmission of at least some of the digitizedseismic data to the recording station, wherein the respective sensormodules are configured to periodically receive from the sub-system acalculated amount of drift of its respective sensor module clockrelative to the high precision clock.
 2. The system of claim 1 adaptedto perform land seismic data acquisition.
 3. The system of claim 1adapted to perform marine seismic data acquisition.
 4. The system ofclaim 1 wherein respective DSPs are configured to upsample digitizedseismic data at a first fixed sampling rate relative to the highprecision clock.
 5. The system of claim 4 wherein the digitized seismicdata is upsampled by the respective DSPs using an interpolationtechnique selected from linear and nonlinear interpolation, based on thecalculated amount of drift, to increase the respective sensor modules'effective sampling rate.
 6. The system of claim 5 wherein the respectiveDSPs are configured to periodically downsample the digitized seismicdata at a second fixed sampling rate relative to the high precisionclock.
 7. The system of claim 1 wherein the respective sensor modulesare configured to periodically receive from the sub-system thecalculated amount of drift of its sensor module clock relative to thehigh precision clock based on a nominal drift of the sensor module'sclock and a level of noise in the system.
 8. The system of claim 1further comprising one or more base stations for receiving the digitizedseismic data from the plurality of sensor modules via first wirelesslinks, wherein the plurality of sensor modules are disposed proximate tothe one or more base stations.
 9. The system of claim 8 wherein at leastone group of sensor modules of the plurality of sensor modules relays atleast some of the digitized seismic data wirelessly within the at leastone group via a communication topology selected from a partial meshtopology, a mesh topology, and a star topology.
 10. The system of claim8 wherein at least one group of sensor modules of the plurality ofsensor modules relays data packets wirelessly within the at least onegroup via multi-hopping.
 11. The system of claim 8 wherein the one ormore base stations are selected from mobile and non-mobile communicationdevices.
 12. The system of claim 8 wherein the first wireless links areselected from wireless personal area network (WPAN) communicationprotocols.
 13. The system of claim 12 wherein the WPAN communicationprotocols are independently selected from protocols covered by IEEEstandard 802.15.
 14. A seismic data acquisition system comprising: oneor more seismic sources; a plurality of sensor modules configured toasynchronously sample seismic data, wherein respective sensor modulescomprise an analog seismic sensor, an A/D converter (ADC) for generatingdigitized seismic data, a digital signal processor (DSP), and a sensormodule clock; a seismic data recording station; and a seismic datatransmission sub-system comprising a high precision clock incommunication with a global positioning system, the sub-system allowingtransmission of at least some of the digitized seismic data to therecording station, wherein the respective sensor modules are configuredto periodically receive from the sub-system a calculated amount of driftof its respective sensor module clock relative to the high precisionclock.
 15. The system of claim 14 wherein respective DSPs are configuredto upsample digitized seismic data at a first fixed sampling raterelative to the high precision clock.
 16. The system of claim 15 whereinthe digitized seismic data is upsampled by the respective DSPs using aninterpolation technique selected from linear and nonlinearinterpolation, based on the calculated amount of drift, to increase therespective sensor modules' effective sampling rate.
 17. The system ofclaim 14 wherein the respective sensor modules are also configured toperiodically receive from the sub-system information sufficient tocalculate the amount of drift.